A number of different methods have been used to hold a lens in place during the surfacing process. The method most commonly used at present makes use of a low-melting-temperature metal alloy to form or attach a "block" to the semifinished surface of a lens "blank." This procedure is often referred to as "lens blocking."
A common low-melting-temperature alloy comprises bismuth, tin, lead, cadmium, indium, and antimony. When these elements are combined correctly, the alloy melts at a temperature considerably lower than any one of its component elements would melt by itself. The alloy will neither cause a plastic lens to melt nor a glass lens to crack. For example, one alloy that melts at 47.degree.C. is made from the following combination of metals: 45% bismuth; 23% lead; 8% tin; 5% cadmium; and 19% indium. This alloy will work for either plastic or glass lenses. In general, a higher melting alloy (e.g., one which melts at 70.degree. C.) will generally only work for glass lenses.
Unfortunately, many of the present metal alloy materials pose significant environmental and health hazards. For example, lead, a common ingredient in many alloys, is considered to be a strong protoplasmic poison which can be introduced into the body through ingestion, inhalation and skin absorption. Similarly, cadmium may also pose significant health hazards. These hazards are particularly acute since many of the procedures used in the ophthalmic laboratory may cause fumes and/or dust particles of these metals to be released to the air, thereby creating environmental and health hazards for those formulating these alloys or those working with them.
A "blocker" is a piece of equipment employed for the purpose of lens blocking. Blockers that use metal alloy either inject molten alloy between the semifinished lens and a preformed block, or mold a block fully and completely from the alloy material. Both types of blockers have a melting pot that is thermostatically regulated, and a heated feeding tube.
Ideally, for plastic lenses, the alloy temperature is kept just above its melting point until it fills in the cavity between the lens and lens block. For example, the 47.degree. C. alloy used for plastic, polycarbonate and polyurethane lenses should preferably be kept at 52.degree. C., or even lower if possible. For glass lenses the alloy temperature may be maintained at higher temperatures (e.g., about 77.degree. C.).
Once on the lens, the alloy should be cooled as rapidly as possible. In addition to saving time, rapid cooling of the alloy helps prevent the formation of aberrations or indentations on the surface of the lens. However, it is also important that the alloy be allowed to cool fully before generating the lens. If a lens is generated too soon after blocking, the alloy may not have cooled evenly, thereby producing surface distortion or waviness or the lens may become dislodged from the blank.
There are two convenient ways used to harden the alloy quickly. The first is to circulate cold running water or a coolant through a water or "chill" ring, which fits around the lens block. This causes the lens block and alloy to chill, "freezing" the alloy. The second method is to chill the block (e.g., in a refrigerator) before it is placed on the blocker.
A tape or other lens coating is often used when surfacing lenses: (1) to prevent the lenses from being scratched; (2) to serve as a heat shield (e.g., to protect a plastic lens from warpage caused by the heat of the alloy); (3) to achieve better or enhanced alloy adherence; and (4) to eliminate the step of cleaning the lens after surfacing. The cleanest and most common method for protecting lens surfaces and holding the block securely is the use of a surface tape. Tapes for this purpose were developed by the 3M Company and may be applied by placing the lens in a small chamber, stretching the tape over the chamber, and applying a partial vacuum. The lens moves up to the tape and the tape is pulled down over the lens surface. Alternatively, lens coatings, available in brush-on and spray applications, may be applied to the convex side of the lens.
Blocks used for glass lenses ("glass blocks" or "glass-lens blocks") are generally about 43 mm in diameter. They do not need to be large, since their purpose is purely to hold the lens during generating. Blocks used for plastic lenses ("plastic-lens blocks") must not only hold the lens in place, they must also keep it from flexing (bending) during generating, fining, and polishing. For that reason, plastic-lens blocks are generally considerably larger than glass-lens blocks. Since blocks for plastic lenses should be made as large as possible for each grinding situation, they are available in a variety of sizes ranging from approximately 55 to 68 mm. Generally, the largest block that can be used on a given semifinished lens blank is chosen. It should be understood that "glass-lens blocks" are generally made from steel, not glass. They are normally used for blocking glass lenses. Neither are "plastic-lens blocks" made exclusively from plastic. Such blocks are normally made from aluminum.
When a lens has been blocked using a metal alloy, it can be deblocked through shock deblocking or hot-water deblocking.
Shock deblocking of plastic lenses is done with a ring that is placed around the outside of the lens block on the front surface of the lens. It is deeper than the block, so when the lens and block are turned block side down, the ring may be struck against a flat surface. The block drops off the lens from the shock.
Hot-water deblocking is a commonly used technique that utilizes a hot-water bath. The temperature of the water is kept below the boiling point. The blocked lenses are placed on a rack, which is lowered into the water. With the lens under water, the alloy begins to melt and drips to the bottom of the tank. There is a valve at the bottom of the tank through which the liquid alloy can be drained off from time to time. After deblocking, the blocks and lenses are removed from the rack, the surface tape removed, and the lenses cleaned.
Attempts have been made to use less toxic materials in place of the toxic metal alloy. For example, materials comprising low molecular weight thermoplastic polymers and resins have been tried. Unfortunately, some of these compositions exhibit generally poor physical properties (e.g., they are relatively brittle and of little cohesive or tensile strength). Others are very soft and waxy (or become soft and waxy during use), and are prone to flexing which could cause lens distortion during processing. In addition, some of these compositions are also very tacky and messy to work with and do not directly adhere well to glass lenses. Cleaning of these tacky compositions from lens blanks can be very time consuming and significantly increase the cost of the lens processing operation. Also, current wax compositions tend to be heat sensitive at temperatures encountered during normal processing conditions (e.g., grinding and polishing). Materials that are sensitive to normal operating temperatures (e.g., those materials that soften significantly at those temperatures) are undesirable for use in this application. Also, some of the wax based materials are not recommended for use in this application. Also, some of the wax based materials are not recommended for use with glass lenses or cannot be used in the presence of petroleum based lubricants.